Theories include communication, neurological function, regulating temperature and a link with stress.

The report said: "Given that the frequency of yawning in our sample of healthy foetuses declined from 24 to 36 weeks gestation, it is possible that yawning and simple mouth opening have a maturational function early in gestation.

"Although yawning and simple mouth opening have the same trajectory shape over age, it is notable that the yawning rate is just over double the non-yawning rate."

Further research was needed to examine whether foetal yawning is related to central nervous system development, the report added.